With a fast development of the electronic technology, fields of communication, navigation, astronautics, measurement and control, high precision measurement and mobile phone and the like have increasingly demands on some indexes, such as short-term stability, long-term stability and aging rate and so on, of a frequency source. Nowadays, with the rapid development of modern communication technology, various communication systems also have increasingly demand on synchronization of time and frequency. Now many frequency outputs with high stability, high accuracy and good aging rate are implemented by using a highly precise phase-locked loop. The fast, highly precise lock of a local output frequency and a nominal frequency can be implemented by using the high precise phase-locked loop. The phase-locked can make each frequency index of a local source in which one of frequency indexes is lower than that of a standard source obtain obvious improvement. Lock speed and precision of the phase-locked loop are a basis on which we measure the indexes of the phase-locked loop. First, the locking precision of the phase-locked loop must be high enough, the higher the phase-locked precision is, the less the frequency difference between the locked source and the standard source is during the frequency locking, and thus the synchronization between systems are implemented more easily. Second, the phase-locked loop must be able to implement the fast locking, the faster the locking speed of the phase-locked loop is, the faster the implementation of locking between the locked source and the standard source is, and the faster the implementation of frequency synchronization between frequency sources of the communication system is, the faster tracking the locked source can be to implement the synchronization.
The phase-locked loop circuit can improve the short-term stability and frequency output precision of the output frequency of the local source, in particular, improvement of the long-term stability thereof is especially obvious. In the fields such as navigation and the like, the phase-locked loop can make a local thermostatic crystal oscillator keep a good short-term stability, and the long-term stability of the local thermostatic crystal oscillator can be basically maintain at the level of an atomic lock in the navigation system because the local thermostatic crystal oscillator keeps the synchronization with the atomic clock in the navigation system all the time. Thus, not only the good short-term stability of the thermostatic crystal oscillator can be maintained, but also the thermostatic crystal oscillator can obtain the better long-term stability which is close to the level of the atomic lock because of locking. Because of locking with the atomic lock, the short-term stability of the thermostatic crystal oscillator can also be improved to a certain degree.
The traditional PLL (phase-locked loop) makes signals fin and fout pass through a complicated frequency conversion circuit by using a frequency divider, converts the signals fin and fout into the same lower frequency signals and then performs a phase discrimination, the principle of which is shown in FIG. 1.
The current phase-locked loop is mainly ranged into two types: an analog phase-locked loop and a digital phase-locked loop. The analog phase-locked loop is the phase-locked loop which is widely applied at the earliest, which has the feature of high phase-locked precision. A low-pass filter and a voltage-controlled oscillator of the analog phase-locked loop circuit are all analog circuits, with disadvantages such as charge drift, components easy to aging, parameters instability.
Before the digital phase-locked loop does not appear, the analog phase-locked loop is widely applied in various fields because of its high phase-locked precision. Nowadays, with a rapid development of digital circuits, more and more fields use the digital circuits, and it can be said that the digital circuits are applied almost everywhere. In some digital circuits, the analog circuit is required to be performed a corresponding digital conversion process when the analog phase-locked loop is used to lock some digital signals, which will increase the complexity of the phase-locked loop circuit, therefore, applications of the analog phase-locked loop in some digital circuits are limited.
The digital phase-locked loop can be applied in the modern digital circuit directly, and the digital signal need not to be firstly converted into the analog signal and then locked by using the analog phase-locked loop. However, the traditional digital phase-locked loop also exists some problems in the following:
1) Long locking time, the traditional digital phase-locked loop controls the thermostatic crystal oscillator by adopting a signal which is frequency-divided at first and then passes a filter using a phase error; it can be seen from a loop formula of the phase-locked loop that, a phase-locked process of the traditional phase-locked loop is an oscillation convergence process, and the locking time is longer because a repetitive process of convergence exists;
2) Phase-locked precision is not high enough; for a full digital phase-locked loop implemented by using a DDS (Direct Digital Synthesizer), the phase-locked precision is not high enough because the phase-locked precision is affected by DDS resolution.
In conclusion, the phase-locked loop is more and more widely applied in the fields such as SDH (Synchronous Digital Hierarchy) communication, navigation and high precision measurement, and so on. Particularly, for the high precision communication field that requires the frequency output of the local source to be consistent with the frequency output of the standard source rapidly, such as the SDH communication, it not only requires to be able to implement the fast locking between the local source and the standard source, but also requires to be able to implement the fast high precise and fast locking